Much attention has recently been directed to materials having nonlinear optical effects, which, when irradiated with a strong laser beam, provide transmitted light having different components from incident light through the mutual interaction therebetween. Such materials are generally known as nonlinear optical materials and are described in detail, e.g., in David J. Williams (ed.), Nonlinear Optical properties of Organic and Polymeric Materials, ACS Symposium Series 233, American Chemical Society (1983), M. Kato & H. Nakanishi (eds.), Organic Nonlinear Optical Materials, CMC Co. (1985), etc.
One of the applications of nonlinear optical materials is a wavelength modulation device utilizing second harmonic generation (SHG) ascribed to the secondary nonlinear effect and sum frequency wave and difference frequency wave. Nonlinear optical materials which have been used practically in this application include inorganic perovskite structures typically exemplified by lithium niobate. In recent years, .pi.-electron conjugated organic compounds having an electron donative group and an electron attractive group have been proved to exhibit greatly improved performance properties as nonlinear optical materials over the aforesaid inorganic materials. However, p-nitroaniline derivatives known to exhibit high SHG performance, such as 2-methyl-4-nitroaniline (MNA), 2-N,N-dimethylamino-5-nitroacetoanilide (DAN), etc., show low transmittances to blue light due to their strong yellow color. Substitution of a carbon atom of the benzene nucleus of nitroanilines by a nitrogen atom, etc., has been studied, but fully satisfactory results are not necessarily obtained. Therefore, it has been desired to develop a nonlinear optical material having a high transmittance of blue light.